The present invention relates to crowbar circuits and, more specifically, to crowbar circuits in DC/DC converters.
By definition, a DC/DC converter accepts a DC input voltage and produces a DC output voltage, typically different from the input voltage. Various types of DC/DC converters are known in the art to step up, step down or invert a DC voltage in applications including, but not limited to, voltage regulators, noise isolators, power supplies, bus regulators, and voltage adapters. For many applications, synchronous DC/DC converters are desirable because they can be programmed, in software and/or hardware, to output ranges of different voltage output levels.
Specifically, synchronous DC/DC converters are used as power supplies for low-voltage integrated circuits. Because the trend in computer architecture is toward small, low-power components capable of operating at very high frequencies, computer circuitry often consists of densely packed integrated components only fractions of a micrometer in size. The sub-micrometer integrated components operate at low-voltages, e.g. less than 2 volts, and require stable, low-noise power supplies. Synchronous DC/DC converters meet the requirements for supplying power to these integrated components.
However, integrated circuitry requiring a stable, low-voltage source can be extremely susceptible to voltage surges. Therefore, power supplies for these low-voltage devices often have some form of over-voltage protection to protect against voltage spikes. A voltage surge within a power supply can arise for many different reasons, e.g. a failed component within the supply or a voltage spike at the supply""s input.
Crowbar circuits provide one form of over-voltage protection for low-voltage DC/DC converters. Typically, a crowbar circuit monitors the level of an output voltage, and if the output voltage rises above a predetermined level, the crowbar circuit reduces the output voltage toward zero. This is often accomplished by triggering a shunt-connected silicon controlled rectifier (SCR) or metal-oxide-semiconductor field effect transistor (MOSFET) thereby shunting output current through the conducting SCR or MOSFET and thus away from the DC/DC converter output. Crowbar circuitry protecting the output of a DC/DC converter usually occupies separate space on a circuit board and, consequently, adds to the cost of the overall circuit.
It is therefore generally desirable to provide a more cost- and space-effective crowbar circuit for use with low-voltage applications. The crowbar should work with low-voltage power supplies, such as synchronous DC/DC converters, without requiring additional power components and heat sinks. The crowbar should also respond quickly to changes in a power supply output voltage so as to protect low-voltage components that is are easily damaged by voltage surges.
The present invention overcomes the disadvantages of previous crowbar circuits by utilizing one or more components that function as both crowbar shunts and power supply rectifier components. Because the crowbar circuit uses switching components already in use by the power supply, the circuit costs less and occupies less space than a conventional crowbar circuit.
More specifically, the circuit comprises an over-voltage detector that monitors the output voltage of a synchronous DC/DC converter. When the output voltage rises above a predetermined threshold, the detector triggers a MOSFET switch already in use as a rectifier in the converter. The trigger signal overrides a periodic switching signal applied to the gate of the MOSFET, causing it to conduct continuously and thereby apply a shunt path for the output current of the rectifier. Because the conducting MOSFET has a small voltage drop and can be turned on quickly, the MOSFET rapidly clamps the DC/DC converter""s output to a voltage low enough to avoid damage to integrated circuitry powered by the converter.